1. Field of the Invention
This invention relates to substance detection using optical systems. More specifically, the invention is a method and system for detecting the presence and/or concentration of a substance in a sample path using polarization-modulated optical path switching and the principles of differential absorption radiometry.
2. Description of the Related Art
Gas filter correlation radiometers (GFCRs) infer the concentration of a gas species along some sample path either external or internal to the GFCR. In many GFCRs, gas sensing is accomplished by viewing alternately through two optical cells the emission/absorption of the gas molecules along the sample path. These two optical cells are called the correlation and vacuum cells. The correlation cell contains a high optical depth of gas species i that strongly absorbs radiation at specific molecular transition wavelengths of the particular gas while passing all other wavelengths. In effect, the correlation cell defines a plurality of spectral notches (i.e., strong attenuation) coincident with the band structure of gas species i. The vacuum cell generally encloses a vacuum or a gas or gas mixture exhibiting negligible or no optical depth, e.g., nitrogen, an inert gas, or even clean dry air. An optical filter (e.g., interference filter) placed in front of the instrument or in front of the detector limits the spectral information to a region coinciding with an absorption band of the gas of interest. The difference in signal strength between these two views of the emitting/absorbing gas species i can be related to the concentration of this gas along the sample path.
A known GFCR for measuring concentration of a single gas is disclosed in U.S. Pat. No. 5,128,797, issued to Sachse et al. and assigned to the National Aeronautics and Space Administration (NASA), the specification of which is hereby incorporated by reference. The GFCR includes a non-mechanical optical path switch that comprises a polarizer, polarization modulator and a polarization beamsplitter. The polarizer polarizes light (that has crossed a sample path after originating from a light source) into a single, e.g., vertically polarized, component which is then rapidly modulated into alternate vertically and horizontally polarized components by a polarization modulator. The polarization modulator may be used in conjunction with an optical waveplate. The polarization modulated beam is then incident on a polarization beamsplitter which transmits light of one component, e.g., horizontally polarized, and reflects light of a perpendicular component, e.g., vertically polarized. The transmitted horizontally polarized beam is reflected by a mirror, passes through a gas correlation cell and on to a beam combiner. The reflected vertically polarized beam passes through a vacuum cell, is reflected by a mirror and is passed on to the beam combiner. The beam combiner recombines the horizontal and vertical components into a single beam which passes through an optical interference filter that limits the spectral content of the incoming radiation to an absorption band of the gas species of interest. The single beam is then incident on a conventional detector. However, this system is limited in that it can only measure a single gas concentration.
A GFCR for measuring multiple gases based on the same optical path switching technique is disclosed in U.S. patent application, Ser. No. 09/019,473, filed Feb. 5, 1998, by Sachse et al. and assigned to the National Aeronautics and Space Administration (NASA). In this system, each optical path contains one or more cells with each cell having spectral features of one or more gases of interest. The two optical paths are then intersected to form a combined polarization modulated beam which contains the two orthogonal components in alternate order. The combined polarization modulated beam is partitioned into one or more smaller spectral regions of interest where one or more gases of interest has an absorption band. The difference in intensity between the two orthogonal polarization components in each partitioned spectral region of interest is then determined as an indication of the spectral emission/absorption of the light beam along the sample path. The spectral emission/absorption is indicative of the concentration of the one or more gases of interest in the sample path.
Both of the afore-described systems require the use of gas correlation cells. However, there are instances where gas correlation cells are not practical. For example, some gases are too dangerous and/or require a gas correlation cell construction that is too expensive for a particular application. Further, some gases such as ozone are too reactive to contain in a gas cell. Still further, it may also be desirable to detect/measure a broad category of gases, e.g., hydrocarbons. However, to accomplish this with a GFCR system, many gases would have to be contained within one cell or the beam would have to be passed through multiple gas cells. This complicates construction and adds to overall system expense. Still further, gas correlation cells are not useful for measuring spectral absorption characteristics of solids or liquids because these substances have broad absorption features.